Nitrogen doped carbon layer coated platinum electrocatalyst supported on carbon nanotubes with enhanced stability

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 1 6 7 7 3 e1 6 7 8 1

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Nitrogen doped carbon layer coated platinum electrocatalyst supported on carbon nanotubes with enhanced stability Quan Zhang 1, Yunfeng Zhang 1, Weiwei Cai, Xinxin Yu, Ying Ling, Zehui Yang* Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences Wuhan, 388 Lumo RD, Wuhan, 430074, China

article info

abstract

Article history:

Enhancement in durability of electrocatalyst is still one of the most important issues in

Received 8 April 2017

polymer electrolyte fuel cells (PEFCs). Here, we report a structurally coated electrocatalyst

Received in revised form

supported on carbon nanotubes (CNT), in which platinum (Pt) nanoparticles are coated by

12 May 2017

nitrogen doped carbon (NC) layers. CNT/NC/Pt/NC shows comparable electrochemical

Accepted 13 May 2017

surface area (ECSA) and oxygen reduction reaction (ORR) activity to the non-coated elec-

Available online 8 June 2017

trocatalyst (CNT/NC/Pt), indicating that NC layer on Pt nanoparticles almost negligibly affects the activities of electrocatalyst; while, CNT/NC/Pt/NC exhibits a higher Pt stability

Keywords:

due to the unique structure, in which the Pt nanoparticles are stabilized by the NC layers

Polymer electrolyte fuel cells

and Pt aggregation is decelerated proved by TEM measurement. Maximum power density

Nitrogen doped carbon layer

of CNT/NC/Pt/NC reached 604 mW cm
2 with Pt loading of 0.1 mgPt cm
2, which only de-

Pt stability

creases by 7% compared to CNT/NC/Pt (650 mW cm
2). The electrochemical analysis and

Fuel cell performance

fuel cell test illustrate that NC layer on Pt nanoparticles enhances the durability without serious deterioration of fuel cell performance. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Sustainable energy sources (fuel cells, solar cells, lithium-ion/ sulfur batteries) have been paid much attention due to the limited fossil fuels [1e5]. Polymer electrolyte fuel cells (PEFCs) have been considered one of the most attractive energy sources for stationary and mobile devices because of its cleanness and high energy conversion efficiency [4,6,7]. Durability of platinum (Pt) electrocatalyst needs to be highly

considered for commercialization of PEFCs since Pt electrocatalyst determines the lifetime of PEFCs. Pt nanoparticles supported on carbon black (CB) is commercially available electrocatalyst for PEFCs; while, CB suffers from carbon corrosion during the real fuel cell operation condition (humidification, high voltage) resulting in loss of Pt nanoparticles; meanwhile, Pt migration and aggregation are unavoidable due to weak interaction between in Pt nanoparticles and carbon supports leading to loss in activity sites of Pt nanoparticles [8,9].

* Corresponding author. Fax: þ86 186 7237 4372. E-mail address: [email protected] (Z. Yang). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.ijhydene.2017.05.099 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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Fig. 1 e Schematic illustration of synthetic procedure of CNT/NC/Pt/NC electrocatalyst.

Alternative carbon supports, carbon nanotubes (CNTs) [10e12] and graphene [13,14], have been widely studied, which exhibit higher resistance toward carbon corrosion due to the highly graphitic structure. As we reported, Pt deposited on poly(vinyl pyrrolidone) (PVP) wrapped CNT showed 15 times higher durability compared to commercial CB/Pt [15]. Thus, replacement of CB by CNTs or graphene was an efficient way to address carbon corrosion problem for Pt electrocatalyst. Accordingly, Pt coalescence needs to be highly considered for improvement of durability. Takenaka et al. systematically studied the effect of silica layer on Pt nanoparticles and an enhanced durability and stable oxygen reduction reaction (ORR) activity was observed after coating with silica layer [16,17]. We also reported that coating Pt nanoparticles with PVP improved the Pt stability [18]. Ohsaka et al. also reported an improved durability after capping Pt nanoparticles with

tantalum oxide (TaOx) layer [19]. Wei et al. reported that Pt nanoparticles embedded in nitrogen doped porous carbon showed higher durability [20]. Not surprisingly, coating Pt nanoparticles with these layers could enhance the durability of Pt electrocatalyst; while, ORR activity or fuel cell performance seriously sacrificed after coating with these layers. Thus, it is desirable to enhance durability of Pt electrocatalyst without sharp deterioration in ORR activity. Based on the above consideration, here, we designed a unique structure for Pt electrocatalyst, in which the Pt nanoparticles were stabilized by the nitrogen doped carbon (NC) layers derived from the carbonization of poly(vinyl pyrrolidone) (PVP) polymer as shown in Fig. 1. NC layer was selected to coat Pt nanoparticles due to the considerable ORR activity of nitrogen atoms. Pristine CNTs were firstly coated by PVP and second PVP layer was formed after Pt deposition. PVP layers

(b)

Intensity

Intensity

(a)

390

0 100 200 300 400 500 600 700 800 Binding Energy / eV

395

400

405

410

Binding Energy / eV

120

(c)

(d)

Intensity

Weight residue / %

100 80 60 40 20

65

70 75 80 Binding Energy / eV

85

0 0

200 400 600 o Temperature / C

800

Fig. 2 e XPS survey scan (a) and narrow scan of commercial CB/Pt (black line), CNT/NC/Pt (blue line) and CNT/NC/Pt/NC (red line) in N1s (b), Pt4f (c) regions, respectively. (d) TGA curves of CNT/NC/Pt and CNT/NC/Pt/NC, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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were converted to NC layers by carbonization under N2 atmosphere. The newly fabricated electrocatalyst was systematically compared to commercial CB/Pt and non-coated electrocatalysts in terms of durability, oxygen reduction reaction (ORR) activity and fuel cell performance; while, it should be note that this study was different from our previous publication, in which Pt nanoparticles were deposited on nitrogen doped carbon nanotubes (N-CNT) and coated with NC layer [21]; while, in this work, pristine CNT was utilized.

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filtration; washing with water and drying at 80  C for 12 h. Pt nanoparticles were deposited on CNT/PVP by the reduction of H2PtCl6 in ethylene glycol aqueous solution at 140  C for 5 h under argon atmosphere. CNT/PVP/Pt was collected by filtration, washing and drying under vacuum at 80  C for 12 h to remove solvent. CNT/NC/Pt was formed by carbonization of CNT/PVP/Pt at 500  C for 3 h under N2 atmosphere.

Synthesis of CNT/NC/Pt/NC

Experimental section Materials and reactants Ethylene glycol (EG), isopropanol, hydrogen hexachloroplatinate hexahydrate (H2PtCl6) and poly(vinyl pyrrolidone) (PVP, K-30) were obtained from Sinopharm Chemical Reagent Co., Ltd. Multi-walled carbon nanotubes (MWNTs) were offered by Timesnano Corp. Commercial CB/Pt electrocatalyst (40 wt%) was obtained from Alfa Aesar. Nafion 212 membrane and Nafion solution (5 wt%) were obtained from Sigma-Aldrich. All aqueous solutions used this work were prepared by using Milli-Q water (18.2 MU cm).

Synthesis of CNT/NC/Pt The pristine CNTs were functionalized by PVP via sonication in water 1 h. The functionalized CNTs were collected by

10 mg of PVP was dissolved in water and 10 mg of CNT/PVP/Pt was dispersed in water by sonication for 10 min. And CNT/NC/ Pt/NC was collected by filtration, washing and carbonization at 500  C N2 atmosphere for 3 h.

Fundamental characterization XPS spectra were recorded by ThermoScientific K-Alpha equipment. The pressure in XPS chamber was maintained under 10
9 Pa during the measurements. Quantitative analysis was performed based on the regions of N1s (395e407 eV) and Pt4f (68e83 eV) with pass energy of 40 eV and linear background. Surface elemental compositions were determined by the ratios of peaks areas. Thermogravimetric analysis (TGA) was conducted by TGA analyzer (NETZSCH STA 449F3) with heating rate of 5  C min
1 and stable oxygen flow. TEM micrographs were taken by JEM-2010 (JEOL, 120 kV) electron microscope.

Fig. 3 e TEM images of commercial CB/Pt (a), CNT/NC/Pt (b) and CNT/NC/Pt/NC (c) electrocatalysts before durability test. (d) HR-TEM image of CNT/NC/Pt/NC.

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0.1

(a)

0 1st 600th 1200th 1800th 2400th 3000th 3600th 4200th

-0.05

-0.1

0

0.2

0.4 0.6 0.8 1 Potential / V vs. RHE

0.05

0

-0.05

-0.1

1.2

(c)

100

Normalized ECSA / %

-1 Pt

0.1

Current density / A mg

(b)

-1 Pt

0.05

Current density / A mg

Current density / A mg

-1 Pt

0.1

0.05

0

-0.05

0

0.2

0.4 0.6 0.8 1 Potential / V vs. RHE

1.2

(d)

90 80 70 60 50

-0.1

0

0.2

0.4 0.6 0.8 1 Potential / V vs. RHE

1.2

0

1000 2000 3000 4000 Number of Cycles

5000

Fig. 4 e Cyclic voltammetry curves of commercial CB/Pt (a), CNT/NC/Pt (b) and CNT/NC/Pt/NC (c) after different potential cycles from 0.6 V to 1.0 V vs. RHE. (d) Normalized ECSAs of commercial CB/Pt (black line), CNT/NC/Pt (blue line) and CNT/NC/Pt/NC (red line) as a function of potential cycle from 0.6 V to 1.0 V vs. RHE. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Electrochemical measurements The electrochemical analysis was tested by a CHI-640e instrument with three-electrode system. Glassy carbon electrode (GCE) with a diameter of 5 mm was used as the working electrode. Ag/AgCl and Pt wire were used as the reference and counter electrodes, respectively. The electrocatalyst ink was typically prepared as follows. The electrocatalyst (1.0 mg) was dispersed in 2.0 mL isopropanol aqueous solution (v/v ¼ 4/1) by sonication and the resultant solution was used to cast on GCE. The Pt loading on GCE was controlled to be 15 mg cm
2. The cyclic voltammetry (CV) curve of the electrocatalyst was obtained in N2-saturated 0.5 M H2SO4 solution after pretreatment. The electrochemical surface area (ECSA) value was evaluated from CV curve based on the hydrogen electro-adsorption peak. All the potentials were changed to the reference hydrogen electrode (RHE). Oxygen reduction reaction (ORR) was measured in O2-saturated 0.5 M H2SO4 solution after ECSA measurement.

Durability test Durability of the electrocatalyst was evaluated based on the load cycle from the Fuel Cell Commercialization Conference of

Japan (FCCJ) [22]. Durability test was simplified to test in 0.5 M H2SO4 electrolyte using a casted GCE. Potential of the GCE was initially kept at 0.6 V vs. RHE for 3 s, then increased to 1.0 V vs. RHE and kept for another 3 s. The procedure was cycled, and after every 600 cycles, CV was carried out to calculate the ECSA (for more details, see Supporting information, Fig. S1).

Fuel cell test Electrocatalyst ink was dispersed in isopropanol aqueous solution (v/v ¼ 4/1) by sonication for 1 h. Continuously, Nafion solution (1.0 mL, 5 wt%) was added and ultrosonicated for 30 min until homogeneous solution was formed. Electrocatalyst ink was filtered on a gas diffusion layer (GDL) to obtain the gas diffusion electrode (GDE). Pt loading on the GDL was controlled to be 0.1 mgPt cm
2. The resultant GDE was dried at 60  C for 12 h to remove isopropanol. Membrane Electrode Assembly (MEA) was fabricated by hot-pressing two GDEs on Nafion® 212 membrane. The size of the MEA was 5 cm2. Before the IeV test, cathode and anode were fed with humidified nitrogen and hydrogen, and then 100 CV cycles (0.05 Ve0.6 V vs. RHE) were carried out to activate the MEAs. Fuel cell performance of the resultant MEAs was measured at

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 2 ( 2 0 1 7 ) 1 6 7 7 3 e1 6 7 8 1

70  C with 100% relatively humidified (RH) gases by fuel cell testing equipment (Model 850e, Scribner Associate, Inc.). IeV and power density curves were tested under the atmospheric pressure and oxygen (flow rate; 0.2 L min
1) and hydrogen (flow rate; 0.1 L min
1) for cathode and anode, respectively.

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respectively, suggesting that carbonization process showed negligible effect on Pt loading. The decrease in Pt content was due to the additional NC layer in CNT/NC/Pt/NC, which was calculated to be 10 wt%.

Electrochemical analysis

Results and discussion Fundamental characterization After carbonization, the synthesized electrocatalysts were measured with XPS as shown in Fig. 2. N1s peaks were detected at 400 eV derived from nitrogen doped carbon layer (NC) layers and CNT/NC/Pt/NC showed an intensive N1s peak due to the double NC layers in the electrocatalyst compared to CNT/NC/Pt as shown in Fig. 2b [23]. Intensity of Pt4f peaks for CNT/NC/Pt was higher compared to CNT/NC/Pt/NC due to the coverage of NC layer on Pt nanoparticles. Quantitative analysis of mass ratio of Pt:N in CNT/NC/Pt/NC was higher compared to that of CNT/NC/Pt as displayed in Table S1. Additionally, Pt4f peaks of CNT/NC/Pt and CNT/NC/Pt/NC positively shifted (0.1 eV, Fig. 2c) with relative to that of commercial CB/Pt (C1s peak was calibrated to 284.5 eV as standard for three samples, Figure S2), which would be to the partial ionization of Pt during the carbonization process. From TGA measurement (Fig. 2d), the Pt contents were determined to 39.9 wt% and 36.0 wt% for CNT/NC/Pt and CNT/NC/Pt/NC,

Before electrochemical analysis, the morphologies of electrocatalysts were observed by TEM as shown in Fig. 3. Commercial CB/Pt (40 wt%) was used as control sample shown in Fig. 3a. Pt nanoparticles were homogeneously deposited on CNT as shown in Fig. 3 (b, c). The NC layer on Pt nanoparticles was clearly observed for CNT/NC/Pt/NC as shown in Fig. 3d, which was calculated to be 1 nm. Pt sizes were 3.3 ± 0.3 nm, 3.0 ± 0.1 nm and 4.0 ± 0.2 nm for commercial CB/Pt, CNT/NC/Pt and CNT/NC/Pt/NC electrocatalysts, respectively (for histograms, see Supporting information, Fig. S2), indicating that Pt nanoparticles were slightly grown during the carbonization process. From the CV curves shown in Fig. 4, hydrogen adsorption/desorption peaks and Pt oxidation/reduction peaks were clearly observed. Electrochemical surface areas (ECSA) can be calculated from CV curve based on following equation: ECSA ¼ Q H =ð210  Pt loading on electrodeÞ

(1)

where, QH is the charge of hydrogen electro-adsorption from 0.05 V to 0.35 V vs. RHE [24,25]. 2
1 2
1 The initial ECSAs were 71.1 m2 g
1 Pt , 91.0 m gPt and 68.5 m gPt for commercial CB/Pt, CNT/CN/Pt and CNT/NC/Pt/NC,

Fig. 5 e TEM images of commercial CB/Pt (a), CNT/NC/Pt (b) and CNT/NC/Pt/NC (c) electrocatalysts after durability test. (d) HR-TEM image of CNT/NC/Pt/NC.

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Table 1 e Parameters of commercial CB/Pt, CNT/NC/Pt and CNT/NC/Pt/NC.

1

Electrocatalyst Pt size (nm) Pt size (nm) EffPt (%) EffPt (%) before after before after

0.8

respectively. Due to the different Pt size, Pt utilization efficiency was calculated to estimate the electrocatalyst based on following equation [26,27]: Theoretical surface area ¼ 6=rd

(2)

Pt utilization efficiency ¼ ECSA=TSA

(3)

0

(a)

400

0.4

300

-2 -3 -4 -5 -6 0

0.2

0.4

0.6

0.8

Potential / V vs. RHE

1

1.2

100 0

0 0

0.5

1

1.5

-1 Pt

-1 -2 -3 -4

0.9

-2

NC layers, which decelerated the Pt migration and aggregation. In order to confirm the deceleration of Pt aggregation, the electrocatalysts were measured by TEM after durability test as shown in Fig. 5. Pt nanoparticles were grown to 5.4 ± 0.5 nm, 4.2 ± 0.4 nm and 4.1 ± 0.5 nm for commercial CB/Pt, CNT/NC/Pt and CNT/NC/Pt/NC, respectively (for histograms, see Supporting information, Fig. S3). Negligible change in Pt size of CNT/NC/Pt/NC evidenced that Pt growth was successfully supressed by two NC layers. Additionally, the NC layer was still observed for CNT/NC/Pt/NC electrocatalyst after durability (Fig. 5d), illustrating that NC layer was electrochemically stable. After durability test, ECSA of CNT/NC/Pt/NC was 64.4 m2 g
1 Pt , which was higher than those of CNT/NC/Pt 2
1 (59.0 m2 g
1 Pt ) and commercial CB/Pt (35.6 m gPt ). As shown in Table 1, Pt utilization efficiency of CNT/NC/Pt/NC remained

0.2

0.85

2

Fig. 7 e IeV and power density curves of MEA fabricated from commercial CB/Pt (black line) and CNT/NC/Pt/NC (red line) electrocatalysts under 70  C with 100%RH. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(b)

-5 0.8

-2

200 0.2

Mass activity / A mg

-1

-7

0.6

0.2

(c)

MA

-2

0

Current density / mA cm

Current density / mA cm

-2

1

500

Current density / A cm

Thus, Pt utilization efficiencies of CNT/NC/Pt (98%) and CNT/NC/Pt/NC (98%) was much higher compared to that of commercial CB/Pt (84%) due to embedded Pt nanoparticles into micropores of CB surfaces. During the Pt deposition, some Pt nanoparticles were loaded into micropores on CB surfaces and Nafion ionomers clogged these micropores, thus, some Pt nanoparticles were unable to be used during the electrochemical measurement. The comparable Pt utilization efficiency between CNT/NC/Pt and CNT/NC/Pt/NC suggested that NC layer on Pt nanoparticles possessed porous structure and Pt nanoparticles could be used during CV test. Subsequently, durability test was carried out by potential cycling from 0.6 V to 1.0 V vs. RHE in order to study the Pt stability. From Fig. 4a, hydrogen adsorption/desorption peaks decreased with the increasing in potential cycles for commercial CB/Pt and remained only 50% of initial ECSA after 4200 potential cycles, which was lower compared to CNT/NC/Pt (ECSA retention: 65%, Fig. 4b). The higher ECSA retention of CNT/NC/Pt was probably due to the partially ionized Pt nanoparticles proved by XPS (Fig. 2c). CNT/NC/Pt/NC showed almost stable CV curves (Fig. 4c) and showed only 4.4% loss in ECSA after same potential cycles (Fig. 4d), which would be due to the unique structure. Pt nanoparticles were stably anchored between two

Power density / mW cm

69 88 94

0.95

Potential / V vs. RHE

1

-2 Pt

84 98 98

SA 0.15

0.15

0.1

0.1

0.05

0.05

0

1

2 Electrocatalyst

3

Specific activity / mA cm

5.4 ± 0.5 4.2 ± 0.4 4.1 ± 0.5

600 Cell voltage / V

3.3 ± 0.3 3.0 ± 0.1 4.0 ± 0.2

CB/Pt CNT/NC/Pt CNT/NC/Pt/NC

700

0

Fig. 6 e (a) Linear scan voltammetry (LSV) curves of commercial CB/Pt (black line), CNT/NC/Pt (blue line) and CNT/NC/Pt/NC (red line) electrocatalysts measured in O2-saturated 0.5 M H2SO4 electrolyte with rotation speed of 1600 rpm. (b) Magnified LSV curves of commercial CB/Pt (black line), CNT/NC/Pt (blue line) and CNT/NC/Pt/NC (red line) electrocatalysts from 0.8 V to 1 V vs. RHE. (c) Mass (red column) and specific (blue column) activities of commercial CB/Pt (1), CNT/NC/Pt (2) and CNT/NC/Pt/NC (3), respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Table 2 e Comparison of single cell voltages @0.5 A cm¡2 from electrocatalysts in recent literatures. Electrocatalyst

Temperature ( C)

Pt loading (cm
2)

Reactant

[email protected] A cm
2 (V)

Ref.

70 70 60 70 80 65 80

0.10 0.30 0.40 0.50 0.20 0.30 0.50

H2/O2 H2/O2 H2/O2 H2/O2 H2/O2 H2/O2 H2/O2

0.69 0.75 0.65 0.62 0.55 0.65 0.62

This work [34] [35] [36] [37] [38] [39]

CNT/NC/Pt/NC PANI@Pt/C Pt/VC Pt/N-graphene Pt/CNT PtPd/CeN Ag@Pt/C

94% after durability test; while, Pt utilization efficiencies of CNT/NC/Pt and commercial CB/Pt decreased to 89% and 69% after durability test, respectively, which was due to the growth in Pt size. Oxygen reduction reaction (ORR) activity was measured in O2-saturated 0.5 M H2SO4 electrolyte (Fig. S4). Diffusionlimited current densities of three electrocatalysts reached 5.5 mA cm
2 (Fig. 6b) which was close to theoretical value, indicating all the electrocatalyst films were homogeneously casted on electrode [28]. From the magnified profiles from 0.8 V to 1.0 V vs. RHE (Fig. 6b), ORR curves of CNT/NC/Pt and CNT/NC/Pt/NC were positively shifted compared to that of commercial CB/Pt, suggesting higher ORR activity. Due to the NC layer on Pt nanoparticles, ORR activity of CNT/NC/Pt/NC was somewhat lower than that of CNT/NC/Pt, which was consistent with the ECSA observation. In order to quantitatively analyzed ORR activity, mass and specific activities were calculated based on following equations [29,30]: 1=i ¼ 1=ik þ 1=id

(4)

MA ¼ ik =ðPt loading on electrodeÞ

(5)

SA ¼ MA=ECSA

(6)

where, i, ik and id were experimental, kinetic and diffusionlimiting currents, respectively. Mass activity @0.9 V vs. RHE (Fig. 6c) of CNT/NC/Pt reached 0.15 A mg
1 Pt , which was higher compared to that of commercial CNT/NC/Pt/NC (0.14 A mg
1 Pt ) due to the higher Pt utilization efficiency; while, CNT/NC/Pt/NC still showed higher ORR activity compared to commercial CB/Pt (0.1 A mg
1 Pt ), which was comparable to the reported values in literature [31,32]. All the three electrocatalysts showed similar specific activities (0.15 mA cm
2 Pt ). The higher mass activity of CNT/NC/Pt/NC with relative to commercial CB/Pt would be due to the higher Pt utilization efficiency since some Pt nanoparticles were useless in CB/Pt electrocatalyst because of the micropores [33].

Fuel cell test Single cell test was measured at 70  C with 100% relatively humidified hydrogen and oxygen flows to anode and cathode, respectively. As shown in Fig. 7, CNT/NC/Pt/NC showed almost similar IeV curves due to the comparable ORR activity measured in half-cell. Maximum power density of CNT/NC/Pt/ NC reached 604 mW cm
2, which was close to CNT/NC/Pt (650 mW cm
2) and 80% higher compared to commercial CB/Pt (345 mW cm
2). The comparable power density illustrated that

the second NC layer on Pt nanoparticles showed negligible effect on the fuel cell performance. In order to make a fair comparison of fuel cell performance, cell voltage @0.5 A cm
2 was take for comparison since fuel cell device is operated under this condition. CNT/NC/Pt/NC showed one of the highest single cell voltages @ 0.5 A cm
2 (0.69 V) among the electrocatalysts listed in Table 2, suggesting CNT/NC/Pt/NC showed higher performance and is utilizable in real fuel cell devices.

Conclusion In conclusion, Pt nanoparticles were successfully coated by the nitrogen doped carbon (NC) layers derived from carbonization of PVP. Coated Pt electrocatalyst showed almost stable Pt size and electrochemical surface area during potential cycling; in contrast, non-coated and commercial electrocatalysts lost 35% and 50% of initial ECSA, respectively. Meanwhile, the Pt utilization efficiency of coated Pt electrocatalyst was stable during the durability test. NC layer on Pt nanoparticles exhibited negligible effect on oxygen reduction reaction (ORR) activity, suggesting that NC layer enhanced Pt stability without deterioration in activity.

Acknowledgement This work is supported by the National Natural Science Foundation of China (No. 21503197) and Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan) (CUG170615).

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.05.099.

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